1. Field of the Invention
This invention relates to a riser sleeve with breaker core for use in the casting of metals.
2. Description of the Prior Art
It is a common practice to use an exothermic or insulating riser sleeve with a ceramic breaker core, a sand breaker core or a breaker core made of heat insulating material attached to its bottom. This is especially true in the case of casting ductile iron, ordinary iron and steel. Although no particular problem is encountered when the riser sleeve is used in this manner for casting iron, in the casting of steels, particularly of special steels, the riser sleeve and breaker core 6 disposed in the sand mold 7 as shown in FIG. 3 before pouring of the melt are found to become as shown in FIG. 4 after pouring of the melt. Specifically, the bottom of the breaker core 6 is distorted upwardly, as indicated by the arrow E. While the attachment of the breaker core thus makes it easier to break off the riser 8, additional steps become necessary for eliminating the distortion occurring under the riser. The advantage of using the breaker core is offset by this disadvantage. Since the breaker core is therefore seldom used in casting special steels, considerable labor and cost is required for riser removal.
Further, if the sand mold is fabricated using a large diameter riser sleeve, as shown in FIG. 5, the sand packing becomes poor at the region indicated by the arrows B. Since this lowers the strength of the sand mold at this region, penetration of the melt is apt to occur.
On the other hand, one property required of the breaker core is that it exhibit excellent breakdown ability during the finish processing following completion of the casting. From this viewpoint, it is preferable to fabricate the breaker core using an organic binding material and, in fact, breaker cores fabricated using thermosetting phenol resin as a binding material are widely employed. Although these neck-down cores do not produce casting defects when used to cast iron and iron alloys, their use in casting steel leads to the occurrence of gas defects in the casting owing to nitrogen gas generated by the binding material. Breaker cores using acid hardening resin as the binding material are also employed. While these breaker cores present no problem as regards breakdown ability during finish processing following completion of the casting, the sulfur contained in the binding material gives rise to SO.sub.2, which also produces gas defects in the casting. Other breaker cores fabricated using linseed oils, tung oil, soybean oil or other such drying oil as the binding material are also in use. These exhibit a fair degree of high-temperature strength and do not produce gas defects in the casting. However, the productivity of the breaker core is poor.
Breaker cores made of silicon sand can be used without any problem for casting iron and other metals with relatively low melting points. However, when used to cast high-melting point steel, they are deformed by the heat and pressure of the cast melt. As a result, bulges are formed on the casting.